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the setup:
Antenna goes into the fold-down adapter, then to the coax through mount. The bed has rhino lining from dealer. The breedlove bed rail mount is insulated by bed lining and covered in black paint. The mount is in the center of the bedrail between the cab and bed. I have a ground from the bottom attachment threads at the mount down to a grounded (verified) screw near the drivers side back body mount. This screw did not go to anything else but was already there. The 18 foot goes from the mount down into the frame, up through the front firewall grommet (with silicone to hold in place and seal) near the steering wheel. I zip-tied plastic loom around the cable where it enters and exits the natural frame holes. Then the cable is routed over the steering shaft to the middle console, to the female-to-female adapter and then 3 foot cable to the swr meter. Then another 3 foot from meter to radio.

the tests:
I have verified at the swr end of the antenna cable there is no conductivity between the center post and external ground thread. I have verified at the swr end of the antenna cable there is conductivity between the threads and the 12v ground the radio plugs into.

Do the 4th generation trucks not have ground connection from frame to body or bed? Other ideas?

I have a Magnum Omegaforce S45HP CB radio and have no problems with the SWR. I am using a Wilson 1000 CB Antenna which is a magnet mount. I have it installed in the center of the roof about 1'-6" from the rear of the cab and run the cable through the rear drivers side door. My SWR is 1.1 to 1.2 to 1. I use a Dosy TFC-3001 SWR meter to measure the SWR.

I think your CB Antenna Fold-Down Adapter is your main problem and your antenna mounting location also is not helping your SWR. The bed of the truck is grounded to the frame, so that is not your problem.

I have a CB Antenna Fold-Down Adapter on my motorcycle and a top loaded 4' Firestik FireFly antenna and the top load adjuster is wound in all the way. My SWR is 1.3 to 1 and I think this is do to the added length of the Fold-Down Adapter.

when dealing with low band like CB you need a ground PLANE not just a grounded mount. ideally the GP should be 1/4 of the wavelength in all directions, which would be larger than the vehicle itself. since that is not possible you want as large a GP as is possible, which is why whips mounted thru the body side or even on top of the cab will out talk anything else. as you improve the "look" of the mount you give up performance.

that out of the way, if your SWR is 6 then you have a bad cable, antenna, or it absolutely is not grounded at all. you should ohm the center pin to the antenna as well as to ground and ohm the ground to each end and chassis ground. don't just look for a reading, look for under .5-1 ohm shield to chassis and almost 0 ohms center pin to center pin.

also, having the antenna so close to the cab will cause reflections back from the cab to the antenna. this is reflected power and RP is just another way of checking SWR. try moving it to one side and at least 12" back from the front corner.

you can get this to work fine, it is just a matter of finding what is the issue and correcting it.

I learned my grounding wire from mount to frame probably wasn't helping much.. So this past weekend I removed that and added 3 inch wide grounding straps (not braided) from the bed to the frame and frame to the cab. I also added a strap from my antenna mount to the bed right next to it with a screw threw the rhino and metal. Now my SWR calibrated on 20 has readings of 2.5 on channel 1 and 3.2 on channel 40. I don't see the cab doing this much bad to swr on such a tall antenna. There is only maybe a foot of the antenna behind the rear window.

I have done additional tests:
A different 18 foot cable yielded same results with same radio/antenna.
A Lil Wilson magnet mount on top of cab (near the door, not centered, not tuned) yielded 1.5 swr. (but I'd prefer to not have a magnet mount)

I will try to check it without the fold-down adapter. What else can I look at if that doesn't help much? Do you think this antenna may have issues? I didn't really want to have to buy another one to test.

If anyone comes across this thread and problem themselves in the future, here's the solution. I moved the antenna closer to the side of the truck on the same front rail. The cab was adding about 1.5 to the swr reading. With the cab blocking 1/4 of the antenna instead of 1/2, it has made a huge difference. SWR is at 1.5 now and usable. The only antenna I would mount in the center would be a whip, which is long enough to get over the cab for 90% of it. Maybe this same antenna mounted a tool box worth away from the cab may give good readings but I don't have a toolbox to test.

sorry to go a little off topic, but where is this grommet you ran the coax thru? i'm about to run some 8ga and would prefer to stay away from the main harness. Or is there anywhere else i can run it

back on topic...i know you'd prefer to stay away from the mag mounts but from experience the wilson 5k is a wicked good antenna i was able to sweep almost 2 mhz and stay under 2.0
with my 257hp i was talking around the world

Whiplash, I read the newer truck models are more cumbersome to deal with the headliner parts. If you do use a mount on the roof, doing the hole closer to the back may allow you to maneuver the coax under the metal if you remove the third brake light during the install.